The role of coated vesicles (CVs) in thyroglobulin transport was examined by studies on incorporation of 35S methionine into thyroid slices and examining the thyroglobulin associated with the membranes. The data do not support a primary role for thyroid CVs in either endocytosis or exocytosis of thyroglobulin. A study of in vivo biosynthesis of clathrin and other coated vesicle proteins from rat liver showed that clathrin heavy chains may be recycled during CV formation. A group of proteins of Mr 100,000-110,000 present in the protein coat of coated vesicles has been shown to be essential for the binding of clathrin to the vesicles. In their absence there was no formation, while adding them back reconstituted the binding nature of clathrin to the vesicles. A protein from the above group has been purified to homogeneity and characterized. It reacts in a stoichiometric ratio with clathrin to form cage-like structures. An intermediate polymer having a sedimentation coefficient of 27 S has been detected in the assembly of clathrin (8 S) to baskets (150 S).